Phylogenetic Taxonomy and Classification of the Crinoidea (Echinodermata)

Home , Actinocrinites, Antedon bifida, Articulata (Crinoidea), Blastozoa, Camerata (Crinoidea), Cladida

Phylogenetic taxonomy and classiﬁcation of the Crinoidea (Echinodermata)

David F. Wright,1,* William I. Ausich,1 Selina R. Cole,1 Mark E. Peter,1 and Elizabeth C. Rhenberg2

1School of Earth Sciences, 125 South Oval Mall, Ohio State University, Columbus, OH 43210, USA 〈[email protected]〉, 〈[email protected]〉, 〈[email protected]〉, 〈[email protected]〉 2Department of Geology, Earlham College, 801 National Road West, Richmond, IN 47374, USA 〈[email protected]〉

Abstract.—A major goal of biological classification is to provide a system that conveys phylogenetic relationships while facilitating lucid communication among researchers. Phylogenetic taxonomy is a useful framework for defining clades and delineating their taxonomic content according to well-supported phylogenetic hypotheses. The Crinoidea (Echinodermata) is one of the five major clades of living echinoderms and has a rich fossil record spanning nearly a half billion years. Using principles of phylogenetic taxonomy and recent phylogenetic analyses, we provide the first phylogeny-based definition for the Clade Crinoidea and its constituent subclades. A series of stem- and node-based definitions are provided for all major taxa traditionally recognized within the Crinoidea, including the Camerata, Disparida, Hybocrinida, Cladida, Flexibilia, and Articulata. Following recommendations proposed in recent revisions, we recognize several new clades, including the Eucamerata Cole 2017, Porocrinoidea Wright 2017, and Eucladida Wright 2017. In addition, recent phylogenetic analyses support the resurrection of two names previously abandoned in the crinoid taxonomic literature: the Pentacrinoidea Jaekel, 1918 and Inadunata Wachsmuth and Springer, 1885. Last, a phylogenetic perspective is used to inform a comprehensive revision of the traditional rank-based classification. Although an attempt was made to minimize changes to the rank-based system, numerous changes were necessary in some cases to achieve monophyly. These phylogeny-based classifications provide a useful template for paleontologists, biologists, and non-experts alike to better explore evolutionary patterns and processes with fossil and living crinoids.

Introduction Given their general significance and broad scientific utility across multiple disciplines of inquiry, it is paramount that the Crinoids are a diverse, long-lived clade of echinoderms with a biological classification of crinoids reflects their evolutionary fossil record spanning nearly half a billion years and are repre- heritage. Numerous emendations and informal suggestions for sented by more than 600 species living in marine ecosystems major taxonomic revisions have been opined over the past today (Hess et al., 1999). The geologic history of crinoids is few decades (e.g., Kelly, 1986; Simms and Sevastopulo, 1993; revealed through a highly complete, well-sampled fossil record Ausich, 1998a, 1998b; Webster and Jell, 1999; Hess and (Foote and Raup, 1996; Foote and Sepkoski, 1999) displaying a Messing, 2011), but the most recent comprehensive revision to complex pageant of evolutionary radiation, extinction, ecologic crinoid classification is the 1978 Treatise on Invertebrate innovation, and morphologic diversification (Ausich and Paleontology (Moore and Teichert, 1978). Since publication of Bottjer, 1982; Ausich et al., 1994; Foote, 1999; Peters and the Treatise, the value of revising rank-based systematic Ausich, 2008; Deline and Ausich, 2011; Gorzelak et al., 2015). classifications to be consistent with phylogenetic hypotheses The spectacular fossil record of crinoids is greatly enriched and and/or the explicit use of phylogenetic taxonomy (sensu de complemented by detailed biologic studies on living species. Quieroz and Gauthier, 1990; Sereno, 1999, 2005) has become These studies facilitate opportunities to synthesize information increasingly common in paleontology (e.g., Smith, 1984, 1994; from fossil and extant forms. For example, comparative studies Holtz, 1996, 1998; Sereno, 1997; Padian et al., 1999; Brochu between fossil and living crinoid species have provided insight and Sumrall, 2001; Carlson, 2001; Carlson and Leighton, 2001; into species ecology and niche dynamics (Meyer and Macurda, Brochu, 2003; Forey et al., 2004; Sereno et al., 2005; Butler 1977; Ausich, 1980; Roux, 1987; Kitazawa et al., 2007; et al., 2008; Kelley et al., 2013). We agree with these authors Baumiller, 2008), established developmental bases for mor- that all named taxa in a biological classification system should phologic homologies (Shibata et al., 2015a), and informed ideally represent clades (i.e., monophyletic groups). The phylogenetic hypotheses (Simms and Sevastopulo, 1993; Rouse development of phylogeny-based classifications is not without et al., 2013). Thus, crinoids form a data-rich model system for difficulties or criticism (e.g., Benton, 2000, 2007). However, we exploring major questions in the history of life. advocate that recent advances in understanding the phylogenetic relationships of major crinoid lineages make the biological classification of the Crinoidea ripe for revision. * Present address: Department of Paleobiology, National Museum of Natural ‘ ’ History, The Smithsonian Institution, P.O. Box 37012, MRC 121, Washington, A great strength of so-called phylogenetic taxonomy is its DC 20013-7012, USA 〈[email protected]〉 potential for increasing nomenclatural stability (de Quieroz and 829

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Gauthier, 1994; Brochu and Sumrall, 2001). Under a of lumping forms together because they are alike in phylogeny-based system of classification, groups of taxa are structure without considering how the likeness arose. organized by their patterns of shared common ancestry rather –F.A. Bather (1898, p. 339) than diagnostic traits. This is a particularly useful aspect of phylogenetic taxonomy: if named evolutionary units are defined Formal scientific description and classification of crinoids began by their history of common ancestry, they do not change if new in 1821 when J.S. Miller recognized fossilized stalked echino- information comes to light that necessitates modification of derms from the “environs of Bristol” as a distinct group. taxonomic diagnoses. For example, new fossil discoveries and/ Although he did not include comatulids in his original concep- or more nuanced understandings of phylogenetic relationships tion of the Crinoidea, he anticipated that they were crinoids: may alter the distribution of synapomorphies among members “The combination of these results with those from the Crinoidea of a clade but do not alter the definition of the clade. Moreover, made me anxious to examine the Comatulae … an animal which by naming taxa on the basis of cladogram topologies, phylo- would be defined with sufficient precision as a Pentacrinus genetic taxonomy can provide a precise definition for groups destitute of the column” (Miller, 1821, p. 127). Further, he previously difficult to diagnose by a unique combination of judged Marsupites ornatus Miller, 1821 (an unstalked crinoid of synapomorphies, such as the Articulata (Simms, 1988; Webster Cretaceous age) to be the ‘link’ between comatulids and his and Jell, 1999; Rouse et al., 2013). To avoid potential instability Crinoidea (Miller, 1821, p. 139). Extant stalked crinoids were in taxonomic nomenclature and/or the proliferation of clade unknown until the mid- to late 1860s, when their discovery names, we advocate that major changes in crinoid systematics during oceanic dredging expeditions provided fodder for early should: (1) be based on well-supported phylogenetic hypotheses debates regarding the efficacy of Darwin’s (1859) then recently inferred using rigorous and repeatable quantitative techniques, proposed theory of natural selection (see Alaniz, 2014; Etter and and (2) employ widely used names and/or names with historical Hess, 2015). Thus, the original description, definition, and precedence if available. diagnosis of the Crinoidea relied entirely on fossil remains. In this paper, we propose a series of stem-based and node- Despite the morphological diversity and deep phylogenetic based clade definitions to help standardize nomenclature for divergences among groups of extant species, the inclusion of crinoid higher taxa. The clade definitions proposed herein are living crinoids with fossil forms has not fundamentally altered informed by a series of recent phylogenetic analyses (Ausich Miller’s (1821) concept. Following subsequent inclusion of et al., 2015; Cole, 2017; Wright, 2017) and represent the first the comatulids and extant stalked crinoids with fossil forms, attempt to classify crinoids using the principles of phylogenetic the Crinoidea has withstood nearly 200 years of scrutiny as a taxonomy (de Queiroz and Gauthier, 1992, 1994). distinct group within the Echinodermata. Although Linnaean classifications lack rigorous criteria for In contrast with their long-term recognition as a clade, the assigning ranks, they can nevertheless provide useful (if coarse) classification of taxa within the Crinoidea has been widely debated reflections of phylogenetic relatedness and divergence among since the nineteenth century (Müller, 1841; Angelin, 1878; taxa, particularly in paleontology (Smith, 1984; Potter and Wachsmuth and Springer, 1897; Bather, 1899; Springer, 1913; Freudenstein, 2005; Jablonski and Finarelli, 2009; Soul and Jaekel, 1918). With few exceptions, debates on crinoid classifica- Friedman, 2015). Given the widespread use of rank-based tion have primarily been based on disagreements over phylo- classifications among invertebrate paleontologists in both alpha genetic affinities among taxa rather than systematic practices taxonomy and paleobiological studies, it is prudent to present a among researchers (see Bather, 1899 for a counter example). phylogenetically informed revision of the rank-based classifi- The intensity of early debates over crinoid classification is best cation of the Crinoidea. These revisions modify the existing epitomized by the frequent yet acrimonious exchanges between Linnaean classification of crinoids to better represent the set of Wachsmuth and Springer (e.g., 1885, 1891, 1897) and Bather nested hierarchies implied by phylogenetic trees (Ausich et al., (e.g., 1898, 1899, 1900). Attempts to resolve these debates among 2015; Cole, 2017; Wright, 2017). nineteenth-century systematists have largely shaped the last ~70 In their review of progress made in crinoid research during years of crinoid research (Ausich and Kammer, 2001). the twentieth century, Ausich and Kammer (2001, p. 1167) In their seminal work Evolution and Classification of stated the “immediate challenge for the [twenty-first century] Paleozoic Crinoids, Moore and Laudon (1943) presented a study of crinoids is to establish a phylogenetic classification for classification that incorporated aspects of both Frank Springer’s the entire class.” It is our hope that the dual classification and Francis Bather’s ideas (see discussion in Ausich and systems presented herein will provide a foundation for future Kammer, 2001). With few modifications, Moore and Laudon’s studies employing phylogenetic nomenclature in crinoid (1943) publication formed the basis of the Treatise on research and promote the use of an improved classification Invertebrate Paleontology (Moore and Teichert, 1978). system among researchers who choose to work with the Following publication of the 1978 Treatise, the classification of Linnaean system. crinoids entered a protracted yet frail era of nomenclatural stability. Although few authors have advanced major revisions or comprehensive modifications, many have voiced contention The dredge and the hammer: a brief history of with the Treatise classification (Kelly, 1982, 1986; Kolata, crinoid classification 1982; McIntosh, 1984, 1986, 2001; Ausich, 1986, 1998a, 1998b; Donovan, 1988; Simms, 1988; Simms and Sevastopulo, The whole history of the attempts to classify the Crinoidea 1993; Brower, 1995; Webster and Jell, 1999; Guensburg and shows … the gradual emancipation from the older habit Sprinkle, 2003; Hess and Messing, 2011; Guensburg, 2012;

Ausich et al., 2015). With the exception of Simms and body plan represents a grade of organization within the more Sevastopulo (1993), these studies have been readjustments of inclusive Pelmatozoa, a clade comprising all blastozoan-grade the Moore and Teichert (1978) classification to accommodate echinoderms and crinoids (including the crown group). rank changes, the addition of new groups, and delineation Although the inclusive group of nominal ‘blastozoan’ taxa is not of clade membership defined by phylogenetic studies of extant monophyletic, there are undoubtedly assemblages of blastozoan species. taxa that do correspond to monophyletic groups (Smith, 1984; The study of extant crinoids remains in the shadow of Sumrall and Wray, 2007; Zamora and Smith, 2011; Sumrall and A. H. Clark, who published more than 100 publications on their Waters, 2012; Zamora et al., 2016). morphology, taxonomy, and classification during the early to Important to this debate are the differences among middle twentieth century (e.g., Clark, 1915, 1921; Clark and researchers with respect to their underlying taxonomic princi- Clark, 1967). The advent and application of molecular phylo- ples and systematic practices (see Smith, 1988). Those who genetic methods to crinoid phylogeny has recently thrown light support the monophyly of the Blastozoa and Crinoidea embrace on relationships among extant species (Cohen et al., 2004; systematic practices that emphasize differences (rather than Hemery et al., 2013; Rouse et al., 2013; Summers et al., 2014). similarities) among taxa, recognize plesiomorphic traits as However, these analyses also point toward the need for exten- taxonomically informative characters, exclude character data sive taxonomic revisions and an improved understanding of from consideration of relationships because of a priori beliefs morphologic traits among living species (Messing and White, regarding the distribution of homoplastic traits, and conflate 2001; David et al., 2006; Roux et al., 2013; Summers et al., sister group hypotheses with ancestor–descendant relationships 2014; Hays et al., 2015). Remarkably, there has been little pre- (e.g., Guensburg and Sprinkle, 2003, 2007; Guensburg, 2012; vious work to combine molecular phylogenetic studies of extant Guensburg et al., 2016). These practices differ considerably crinoids with paleontologic data to assemble a more complete from those that infer the Pelmatozoa as a clade. These workers picture of post-Paleozoic crinoid evolutionary history. Efforts to tend to emphasize similarities (rather than differences) among integrate these rich sources of information present both chal- taxa, minimize a priori assumptions regarding hypotheses of lenges and opportunities for future researchers to resolve pat- character evolution, and utilize the principles of phylogenetic terns and processes shaping the crinoid tree of life (Lee and systematics to rigorously test whether apparent similarities in Palci, 2015; Pyron, 2015). form reflect synapomorphies or homoplasy (e.g., Sumrall and Waters, 2012; Sumrall, 2014; Ausich et al., 2015). Given the Crinoid origins and classification recent advances in homology assessment among pentaradiate echinoderms (e.g., Sumrall, 1997, 2008, 2010, 2014; Sumrall Extant echinoderms include the Crinoidea, Echinoidea, and Waters, 2012; Kammer et al., 2013) and computational Ophiuroidea, Asteroidea, and the Holothuroidea, with the latter phylogenetic analyses of echinoderm taxa based on a large four comprising the Eleutherozoa. Although it has been long ensemble of characters, it is becoming increasingly clear that a established that crinoids form the sister group to the Eleuther- blastozoan-grade taxon likely forms the closest immediate out- ozoa, the relationships among many fossil and extant echino- group to the Crinoidea (Kammer et al., 2013; Sumrall, 2014). In derm groups are controversial (Paul and Smith, 1984; Sumrall the future, new developments in phylogenetic research along 1997; David et al., 2000; Smith, 2005; Pisani et al., 2012; with a continued search for the oldest ‘crinoid’ fossils will Telford et al., 2014; Zamora and Rahman, 2014; Feuda and continue to play a role in uncovering the sequence of morpho- Smith, 2015; Reich et al., 2015). The phylogenetic position of logic transitions behind the assembly of the crinoid body plan. crinoids within the Echinodermata was contested throughout the Despite desultory disagreements regarding crinoid origins late twentieth century, with a focal question whether the (Sprinkle, 1973; Ubaghs, 1978; Donovan, 1988; Ausich, 1998a, Pelmatozoa (i.e., stalked echinoderms including blastozoans 1998b; Ausich and Babcock, 1998; Guensburg and Sprinkle, and crinoids) and/or the Blastozoa are monophyletic groups or a 2007, 2009; Guensburg, 2012; Kammer et al., 2013; Ausich ‘grade’ of body plan organization. This is a fundamental ques- et al., 2015; Guensburg et al., 2016), there is nevertheless con- tion not only for understanding the origin of crinoids but also for siderable agreement among workers regarding the pattern of resolving phylogenetic relationships among clades within the branching relationships within the crinoid ingroup. For exam- Echinodermata. One hypothesis of crinoid origins postulates ple, the recent phylogenetic analyses of Guensburg (2012) and that crinoids and blastozoan echinoderms independently Ausich et al. (2015) reveal highly congruent patterns of evolved pelmatozoan-grade body plans (e.g., Sprinkle, 1973, branching relationships among crinoid higher taxa despite the 1976; Mooi and David, 1998, 2008; David et al., 2000; use of alternative outgroups, different data sets, and alternative Guensburg and Sprinkle, 2003; Guensburg, 2012). This interpretations of homologous morphologic characters. We hypothesis proposes that blastozoans and crinoids each com- surmise this growing consensus stems from the improved prise distinct monophyletic groups. By contrast, an alternative taxonomic sampling of the oldest known crinoids (Guensburg hypothesis postulates that blastozoans and crinoids are members and Sprinkle, 2003, 2009; Guensburg, 2010) and implementa- of an inclusive pelmatozoan clade, with crinoids nested within a tion of more rigorous quantitative approaches to testing phylo- paraphyletic Blastozoa (Leuckart, 1848; Bather, 1899, 1900; genetic hypotheses (Guensburg, 2012; Ausich et al., 2015; Cole, Paul and Smith, 1984; Smith, 1984; Paul, 1988; Smith and Jell, 2017; Wright, 2017). 1990; Smith, 1994; Sumrall, 1997; Ausich, 1998a, 1998b; We conclude that congruence observed among tree topo- Clausen et al., 2009; Zamora and Smith, 2011; Kammer et al., logies obtained from researchers with different perspectives 2013; O’Malley et al., 2016). In this hypothesis, the blastozoan indicates strong support for these patterns. Although questions

surrounding crinoid origins remain, this debate is moot with interesting to note that divergence time estimation based on respect to the phylogeny-based definitions and classification relaxed molecular clock models suggests the split between iso- presented herein and ultimately has no bearing on the focus and crinids and other extant groups took place some 231–252 conclusions of this paper. million years ago (Rouse et al., 2013). Thus, molecular phylogenetic analyses and paleontological evidence are in Toward a phylogenetic classification of the Crinoidea general agreement regarding an ancient origin of the crinoid crown group. From the perspective of their geologic history, crinoids are a A summary tree based on results presented by Rouse et al. bottom-heavy clade (Gould et al., 1987). In contrast to the tre- (2013), Ausich et al. (2015), Wright (2017), and Cole (2017) is mendously diverse assemblage of stem lineages, comparatively depicted in the form of a simplified cladogram in Figure 2. few species are encompassed within the crown group (Fig. 1). This cladogram is annotated with the clade names we propose Because of the enormous diversity of the stem group relative to below. Terminal taxa in the cladogram were carefully chosen to the crown group, fossil crinoids have received much systematic maximize stability in phylogenetic nomenclature (Table 1). attention compared to their extant representatives (but see Clark, Sereno (2005) listed numerous criteria for choosing taxon 1915; David et al., 2006; Hess and Messing, 2011; Hemery specifiers in clade definitions. These recommendations include et al., 2013; Rouse et al., 2013). Aside from a number of smaller choosing specifiers that are nested rather than basal (if possible), studies examining relationships among species of middle to late represented by well-known or readily available material, and Paleozoic genera (e.g., Gahn and Kammer, 2002; Kammer and using multiple specifiers where necessary to accommodate Gahn, 2003; Ausich and Kammer, 2008), most investigations of phylogenetic uncertainty and/or alternative hypotheses. We crinoid phylogeny have focused on discerning relationships have carefully chosen our clade definitions to not hinge on labile among Ordovician taxa (Brower, 1995; Ausich 1998b; phylogenetic hypotheses or specific interpretations of unusual Guensburg, 2012; Ausich et al., 2015; Cole, 2017). The Ordo- and/or problematic taxa. vician Period represents a key interval in crinoid evolution Classesofcladedefinitions used in phylogenetic taxonomy because species belonging to various groups of traditionally and their graphical representations used herein closely follow named taxa first appear in rocks of the Lower Ordovician Sereno (1999, 2005). Node-based clade definitions circumscribe (Tremadocian) (Guensburg and Sprinkle, 2003, 2009; the most recent common ancestor of at least two taxa and Guensburg, 2010) and the majority of well-studied groups had all of its descendants. Thus, node-based definitions form the least originated prior to its close. inclusive clade containing a minimum of two specifiers. The divergence between camerate and non-camerate By contrast, stem-based definitions circumscribe the most inclu- lineages forms a fundamental, early split in the history of crinoid sive clade containing at least one internal specifier. In both cases, evolution (Jaekel, 1918; Donovan, 1988; Guensburg, 2012; additional precision is obtained by identifying external specifiers Ausich et al., 2015; Cole, 2017; Wright, 2017) (Fig. 1). For falling outside the clade (i.e., the outgroup). For example, a stem- example, in the recent phylogeny of Ausich et al. (2015), taxa based definition for hypothetical Clade A with two internal and belonging to the Camerata (sensu Moore and Teichert, 1978) one external taxon specifiers can be stated as ‘all species sharing a form the sister clade to all other crinoids, including the proto- more recent common ancestor with species X and Y than Z,’ where crinoids (Guensburg and Sprinkle, 2003). Disparids were X and Y are internal taxon specifiers and Z is an external specifier. recovered as sister to a clade comprised of most ‘cladid’ taxa, In other words, Clade A is stem-defined as the most inclusive clade and hybocrinids were recovered as sister to a group of containing X and Y but not Z. Note the presence of one species as ‘cyathocrine’ cladids (sensu Moore and Teichert, 1978). an external specifier effectively eliminates the entire clade to A similar pattern was recovered by Guensburg (2012, fig. 2). which it belongs. By definition, a clade cannot contain an ancestor Building on these studies, Cole (2017) further assessed the of its sister group. basal split between camerates and non-camerates and tested In phylogenetic taxonomy, clade membership is not the taxonomic status of the Monobathrida and Diplobathrida determined by the presence or absence of a ‘key’ morphologic (Fig. 1). Wright’s (2017) analysis of relationships among non- feature unless that apomorphy (or set of apomorphies) is listed camerate crinoids offers a more nuanced perspective of this in the definition as a qualifying clause (Sereno, 2005). We avoid portion of the crinoid tree than previously recovered. Notably, apomorphic qualifiers in our definitions for several reasons. many so-called Ordovician clades of Guensburg (2012) and First, incomplete preservation may lead to cases where it is Ausich et al. (2015) do not retain their status of monophyly unknown whether a fossil species has the key feature diagnostic when post-Ordovician taxa are considered (Wright, 2017). of the clade in question. Thus, the inclusion or exclusion of a Recent molecular phylogenetic studies indicate broad fossil species depends on character state optimizations rather relationships among major clades of extant crinoids are also than direct data. Second, a trait may be ‘absent’ in a taxon either reaching a consensus, with the Isocrinida representing the sister because it was truly absent or because it was secondarily lost. clade to all other extant crinoids (Rouse et al., 2013, 2015). It is Similarly, a trait may be ‘present’ because of convergent

Figure 1. Taxa representing major crinoid clades: (1) Pentacrinites fossilis Blumenbach, 1804, articulate, from Goldfuss (1831); (2) Taxocrinus colletti White, 1881, ﬂexible, from Springer (1920); (3) Actinocrinites jugosus (Hall, 1859), monobathrid camerate, from Wachsmuth and Springer (1897); (4) Synbathocrinus swallovi Hall, 1858, disparid, from Wachsmuth and Springer (1897); (5) Dendrocrinus caduceus Hall, 1866, eucladid, from Meek (1873); (6) Hybocystites eldonensis Parks, 1908, hybocrinid, from Springer (1911); (7) Porocrinus shawi Schuchert, 1900, porocrinid, from Kesling and Paul (1968); (8) Archaeocrinus microbasalis (Billings, 1857), diplobathrid camerate, from Wachsmuth and Springer (1897). Scale bars = 0.5 cm and applicable as indicated.

evolution. Moreover, stem group taxa commonly have highly Remarks.—This definition captures J. S. Miller’s (1821) origi- heterogeneous distributions of apomorphic traits, which may nal concept based on fossil specimens and retains the name lead to instability when new taxa are sampled and/or alternative ‘Crinoidea’ as the clade comprising the crown group plus all topologies are equally likely. Finally, the timing of a divergence extinct species sharing a more recent common ancestor with a event may not correspond with the acquisition of a diagnostic living crinoid than any echinoderm taxon listed in the preceding apomorphy. For example, the blastozoan Macrocystella is widely as external specifiers (Fig. 2). Further, this definition closely recognized as a basal glyptocystitoid rhombiferan even though it resembles the traditional use and taxonomic content of the Cri- lacks the respiratory structures traditionally ‘diagnostic’ of the noidea as used by both biologists and paleontologists (Bather, Glyptocystida (Paul, 1968; Sprinkle, 1973; Zamora et al., 2016). 1899; Clark, 1915; Jaekel, 1918; Moore and Teichert, 1978; All of these considerations are highly important when considering Hess et al., 1999; Rouse et al., 2013) and accommodates the patterns of character evolution but may lead to nomenclatural current state of uncertainty regarding their nearest extinct sister instability if incorporated into clade definitions. group. In the interest of preserving the taxonomic content and Although we avoid the use of apomorphies to define clades, common meaning of a widely used name, our Clade Crinoidea we do discuss morphological traits potentially useful for taxo- is preferred over Sumrall’s (1997) similarly defined Crinoido- nomic diagnoses. In some cases, our proposed clade definitions formes (see Cantino and de Queiroz, 2010, p. 42). The Crinoi- retain much of their traditional meaning and taxonomic content, dea is comprised of two major clades, the Camerata and the with constituent taxa sharing numerous synapomorphies that form Pentacrinoidea, reflecting the early divergence between cam- unambiguous taxonomic boundaries (e.g., the Flexibilia). erate and non-camerate crinoids (Jaekel, 1918; Donovan, 1988; However, in other cases, either substantial revision was necessary Guensburg, 2012; Ausich et al., 2015). Because we provide the and/or a list of unambiguous diagnostic characters was difficult or Crinoidea with a stem-based definition, the discovery of stem- impossible to obtain (e.g., the Articulata). These challenges ward fossils is accommodated within this definition. highlight the utility of phylogenetic taxonomy. For example, Internal taxon specifiers were chosen because they were many authors have remarked that the Articulata has lacked a included in Miller’s (1821) original description and represent concise, unambiguous definition since it was first erected by well-known, well-preserved, and highly nested members of Miller nearly 200 years ago (Simms, 1988; Webster and Jell, their respective subclades. In contrast to the internal taxon 1999; Hess and Messing, 2011; Rouse et al., 2013). A phylo- specifiers, the choice of external specifiers is more complex. The genetic definition of the Articulata provides a clearer criterion for use of external specifiers in this definition spanning various clade membership and results in a framework for future phylo- ‘blastozoan’ and edrioasteroid-grade groups reflects the current genetic research assessing relationships among hypothesized stem difficulty involved in postulating the nearest definitive sister clades, crown group synapomorphies, and subsequent morpho- group as well as the uncertain state of relationships among logic transitions among crown group subclades. extinct stemmed echinoderms (Smith, 1984; Sumrall, 1997, The clade definitions and revised classification proposed 2014; Ausich, 1998a, 1998b; Guensburg and Sprinkle, 2009; herein represent the present state of knowledge, but systematics Kammer et al., 2013; Ausich et al., 2015; Guensburg et al., is a dynamic science and taxonomic theories are commonly 2016; O’Malley et al., 2016). reinterpreted in light of new discoveries. We fully expect our The analysis of Ordovician crinoids by Ausich et al. (2015) definitions to be refined and/or modified as more information took a conservative approach to outgroup selection by sampling becomes available. Some places of the crinoid tree still require broadly across taxa nested within the Clade Pelmatozoa extensive taxonomic revisions, such as upper Paleozoic (Kammer et al., 2013; Sumrall, 2014). Similarly, we have ‘cladids’ (sensu Moore and Laudon, 1943) and stem articulates chosen species from multiple pelmatozoan groups as external (Wright, 2015b). Despite these potential vicissitudes in the specifiers to help provide nomenclatural stability in the presence taxonomic content and/or definitions within our proposed clas- of phylogenetic uncertainty. Other taxa hypothesized to sification, we agree with G.G. Simpson’s sentiment: “It is represent the crinoid sister group include the stylophorans pusillanimous to avoid making our best efforts today because (David et al., 2000) and edrioasteroids (Guensburg and they may appear inadequate tomorrow” (1944, p. xxx [sic]). Sprinkle, 2009; Guensburg et al., 2016). Stylophorans have long been considered non-radiate stem group echinoderms Systematic paleontology (e.g., Paul and Smith, 1984; Smith, 1984, 2008) and have been cogently demonstrated to lack crown group synapomorphies Crinoidea Miller, 1821 (Smith, 2005). Thus, we do not consider the stylophoran hypothesis further. Guensburg and Sprinkle (2009) and Definition.—The Crinoidea is stem-defined as the most Guensburg et al. (2016) regard edrioasteroid echinoderms, such inclusive clade containing Rhodocrinites verus Miller, 1821, as the stromatocystidid Cambraster or the edrioblastoid Actinocrinites triacontadactylus Miller, 1821, and Pentacrinites Cambroblastus, to possess apomorphies indicating they share fossilis Blumenbach, 1804 but not Rhopalocystis detombesi a more recent common ancestor with crinoids than with other Ubaghs, 1963, Echinosphaerites aurantium (Gyllenhaal, echinoderms. Although this hypothesis contrasts with previous 1772), Eumorphocystis multiporate Branson and Peck, 1940, studies regarding edrioasteroids as stem group eleutherozoans Protocrinites ouiformis Eichwald, 1840, Cheirocystis antiqua (Paul and Smith, 1984; Smith 1984, 1985, 1990; Smith and Paul, 1972, Glyptocystella loeblichi (Bassler, 1943), Cambraster Zamora, 2013), recent investigations suggest that edrioasteroids cannati Miquel, 1894, and Cambroblastus enubilatus Smith and may comprise a para- or polyphyletic group (Kammer et al., Jell, 1990. 2013; Zamora, 2013; Zamora and Rahman, 2014). Some

Figure 2. Cladogram depicting phylogenetic relationships among species used to define major clades within the Crinoidea. Terminal tips correspond to species listed in Table 1. Clades given stem-based definitions are indicated with a downward-facing arrow; clades given node-based definitions are indicated with a circle. Note that many clades named are nested inside other more inclusive clades. Graphical notation of stem- and node-defined clades follows Sereno (2005).

Table 1. Species name, least inclusive clade, and ﬁrst appearance interval for each taxon depicted in Figure 2.

Species Least Inclusive Clade First Occurrence of Species Actinocrinites triacontadactylus Miller, 1821 Monobathrida Mississippian (Tournaisian) Adelphicrinus fortuitus Guensburg and Sprinkle, 2003 Camerata Ordovician (Tremadocian) Alphacrinus mansﬁeldi Guensburg, 2010 Disparida Ordovician (Tremadocian) Antedon biﬁda (Pennant, 1777) Articulata Recent Apektocrinus ubaghsi Guensburg and Sprinkle, 2009 Pentacrinoidea Ordovician (Tremadocian) Archaeocrinus lacunosus (Billings, 1857) Diplobathrida Ordovician (Katian) Carabocrinus radiatus Billings, 1857 Porocrinida Ordovician (Sanbian) Cupulocrinus heterocostalis (Hall, 1847) Flexibilia Ordovician (Katian) Dendrocrinus longidactylus Hall, 1852 Eucladida Silurian (Wenlockian) Endoxocrinus parrae (Gervais, 1835) Articulata Recent Eknomocrinus wahwahensis Guensburg and Sprinkle, 2003 Camerata Ordovician (Tremadocian) Glyptocrinus decadactylus Hall, 1847 Monobathrida Ordovician (Katian) Hybocrinus conicus Billings, 1857 Hybocrinida Ordovician (Sanbian) Hybocystites problematicus Wetherby, 1880 Hybocrinida Ordovician (Katian) Pentacrinites fossilis Blumenbach, 1804 Articulata Triassic (Anisian) Porocrinus conicus Billings, 1857 Porocrinida Ordovician (Katian) Rhodocrinites verus Miller, 1821 Diplobathrida Mississippian (Tournaisian) Rosfacrinus robustus Le Menn and Spjeldnaes, 1996 Eucamerata Ordovician (Katian) Synbathocrinus conicus Phillips, 1836 Disparida Mississippian (Tournaisian) Taxocrinus macrodactylus (Phillips, 1841) Flexibilia Devonian (Famennian)

edrioasteroids, such as the isorophids, may be closely related crinoids (Kammer et al., 2013; Zamora et al., 2013; Zamora and to gogiid eocrinoids, whereas other edrioasteroids, such as Rahman, 2014). Because a comprehensive, up-to-date phylogeny Cambraster, may be closer to glyptocystitoid blastozoans and of pentaradiate echinoderm lineages is currently lacking, we

tentatively follow Guensburg and Sprinkle (2009) and Guensburg narrower restrictions on clade membership to render these taxa et al. (2016) by including both Cambraster and the edrioblastoid monophyletic. Following revision, the Monobathrida and Diplo- Cambroblastus as additional external taxon specifiers. bathrida are sister clades that together comprise the more inclusive Identifying synapomorphies of the Clade Crinoidea Eucamerata (Cole, 2017). Thus, the stem-based definition of the requires a phylogenetic hypothesis of their position within the Camerata contains the Clade Eucamerata and their stem taxa, broader echinoderm clade. As discussed above, this remains including representatives of the oldest known crinoid fossils an open question. Basal members of both the Camerata and (e.g., Eknomocrinus, Cnemecrinus), and genera placed within the Pentacrinoidea have a dicyclic calyx with an irregular field of problematic Reteocrinitidae (see Cole, 2017), and may or may not plates intercalating between fixed proximal brachials, suggest- contain the protocrinoids (see Guensburg and Sprinkle, 2003; ing these may be plesiomorphic traits (cf. Apektocrinus, Guensburg, 2012; Ausich et al., 2015; Cole, 2017). Cnemecrinus, Glenocrinus) (Guensburg, 2012, Ausich et al., 2015; Cole, 2017; Wright, 2017), but a definitive list of shared Eucamerata Cole, 2017 derived traits cannot be provided here. Moreover, it is challenging to propose a list of apomorphies that unambigu- Definition.—The Eucamerata is node-defined as the least ously differentiate crinoids from other echinoderm taxa because inclusive clade containing Actinocrinites triacontadactylus many traits are not exclusive to crinoids. Crinoids have been Miller, 1821, Rhodocrinites verus Miller, 1821, and Rosfacrinus traditionally recognized as distinct from blastozoan-grade robustus Le Menn and Spjeldnaes, 1996. echinoderms in having true ‘arms,’ where arms are defined as coelomic extensions of the body cavity (Sprinkle, 1973). Remarks.—Cole (2017) revised the Monobathrida and Diplo- However, morphologic observations of solute and diploporitan bathrida to represent monophyletic groups while attempting to echinoderms such as Eumorphocystis and the discovery of preserve the greatest number of taxa traditionally included within various Cambrian ‘blastozoans’ with arm-like appendages each (Moore and Teichert, 1978). The name ‘Eucamerata’ strongly suggest that arms may not be an apomorphy unique was proposed to identify the clade of camerates comprised to crinoids (Clausen et al., 2009; Zamora and Smith, 2011; of the sister groups Monobathrida and Diplobathrida, which Sumrall, 2014; Zamora and Rahman, 2014). necessarily excludes stem taxa such as Cnemecrinus and We anticipate future phylogenetic research will help Reteocrinus (Cole, 2017). The Eucamerata comprise the resolve these broader issues in echinoderm phylogeny and majority of camerate taxa and span the Ordovician through evolution. Improved knowledge of relationships among extinct Permian. Eucamerates are characterized generally by the traits pentaradiate echinoderms may also help refine our definition of listed above for the Camerata, but differ in typically having the Clade Crinoidea by removing pleonastic external specifiers. more strongly ankylosed calyx plate sutures, primaxils on the We await its refinement. second primibrachial, holomeric stems, and pinnulate arms (cf. Actinocrinites and Rhodocrinites with Eknomocrinus and Camerata Wachsmuth and Springer, 1885 Reteocrinus). Definition.—The Camerata is stem-defined as the most inclu- In an attempt to preserve the stability of sister group sive clade containing Actinocrinites triacontadactylus Miller, relationships between monobathrid and diplobathrid clades, we 1821 and Rhodocrinites verus Miller, 1821 but not Pentacri- provide a node-based definition for the Eucamerata and stem- nites fossilis Blumenbach, 1804. based definitions for the Monobathrida and Diplobathrida. The internal taxon specifiers Actinocrinites and Rhodocrinites are Remarks.—Camerate crinoids represent a diverse, morphologically highly nested constituents of their respective monobathrid and distinct ‘stem clade’ (sensu Sereno, 1999, 2005) ranging from the diplobathrid subclades (Moore and Laudon, 1943; Cole, 2017). Lower Ordovician to Permian and contain all taxa traditionally Rosfacrinus is cautiously included as an additional external placed within the Diplobathrida and Monobathrida (Moore and specifier because it occupies a somewhat uncertain position at Teichert, 1978; Cole, 2017). Camerates are most easily differ- the base of the eucamerate tree (see discussion in Cole, 2017). entiated from pentacrinoids in having calyx plates united by rigid sutures, a heavily plated tegmen surface covering the Monobathrida Moore and Laudon, 1943 mouth, and a medial plate (or series of plates) in the posterior (i.e., CD) interray. Unlike pentacrinoids, the camerate posterior Definition.—The Monobathrida is stem-defined as the most plate series has no proximal topographic affinity with the C ray, inclusive clade containing Glyptocrinus decadactylus Hall, although some camerate posterior plates may be homologous 1847 and Actinocrinites triacontadactylus Miller, 1821 but not with those of pentacrinoids (see Jaekel, 1918, p. 46; Moore and Rhodocrinites verus Miller, 1821 and Archaeocrinus lacunosus Laudon, 1943; Brower, 1973, p. 301–304; Guensburg and (Billings, 1857). Sprinkle, 2003). In addition, typical camerate species have fixed proximal brachials, interradials, and sometimes intrabrachials, Remarks.—When revising Bather’s (1899) polyphyletic divi- whereas most derived pentacrinoid clades lack these features. sion of crinoids into the Monocyclica and Dicyclica, Moore and Multiple studies indicate strong support for camerate Laudon (1943) placed all camerates with monocyclic calyces monophyly (Ausich, 1998b; Ausich et al., 2015; Cole, 2017). into the Monobathrida. Cole’s (2017) phylogenetic analysis of However, Cole’s (2017) analysis of Ordovician camerates did Ordovician camerate crinoids indicates a strict adherence to not find support for a strict division between monocyclic and Moore and Laudon’s (1943) concept of the Monobathrida is not dicyclic forms. Cole’s (2017) phylogenetic revision proposed monophyletic. However, removal of the stemward camerates

Eknomocrinus and Adelphicrinus renders the Monobathrida a but not Rhodocrinites verus Miller, 1821 and Actinocrinites clade (Cole, 2017). The internal and external specifiers defining triacontadactylus Miller, 1821. this stem-based clade ensure the taxonomic content closely matches Moore and Laudon (1943). Remarks.—The name ‘Pentacrinoidea’ originates from Jaekel’s Monobathrids are a taxonomically diverse group of (1894, 1918) prescient observation that camerate and non- camerates ranging from the Ordovician to Permian and are camerate crinoids form distinct clades. Although authors after traditionally diagnosed as monocyclic camerates. Although Jaekel (1918) did not adopt this name in subsequent classifica- other clades had similar trends in circlet reduction (e.g., the tions (see Lane, 1978; Ausich and Kammer, 2001), Jaekel’s Disparida), the transformation from a dicyclic to monocyclic usage coincides with this strongly supported clade (Guensburg, calyx likely represents a veritable synapomorphy of mono- 2012; Ausich et al., 2015; Cole, 2017; Wright, 2017). Thus, we bathrid camerates, as the dicyclic crinoid Gaurocrinus was propose to reinstate the name Pentacrinoidea with the preceding recovered as the sister taxon to the Monobathrida by Cole definition. (2017). Additional features diagnostic of a typical monobathrid We have chosen two phylogenetically distant non-camerate species include having radial plates larger than other calyx species as internal specifiers. Pentacrinites fossilis is a well- plates, an upright basal circlet, an uninterrupted radial circlet known fossil species from rocks of Jurassic age and is closely (except in the posterior interray), and a posterior interray with related to extant isocrinid crinoids (David et al., 2006), placing it anitaxis plating and an anitaxial ridge. within the Crown Crinoidea (see Articulata below). The species Apektocrinus ubaghsi is a Lower Ordovician fossil and ranks Diplobathrida Moore and Laudon, 1943 among the stratigraphically oldest known crinoids (Guensburg and Sprinkle, 2009). However, all phylogenetic research Definition.—The Diplobathrida is stem-defined as the most indicates it is closer to non-camerates than to camerates and inclusive clade containing Archaeocrinus lacunosus (Billings, diverges stemward of other basal ‘cladid’ (sensu Moore 1857) and Rhodocrinites verus Miller, 1821 but not Actinocri- and Teichert, 1978) taxa such as Aethocrinus (Guensburg and nites triacontadactylus Miller, 1821 and Glyptocrinus Sprinkle, 2009; Guensburg, 2012; Ausich et al., 2015; Wright, decadactylus Hall, 1847. 2017). Our stem-based definition recognizes Jaekel’s (1918) priority of this concept and effectively places all known Remarks.—Similar to the discussion above, Moore and non-camerate species within the Pentacrinoidea. Laudon (1943) placed all of Bather’s (1899) dicyclic camerate Pentacrinoids are a spectacularly diverse and morphologi- crinoids within the Diplobathrida. As with the monobathrids, cally heterogeneous clade ranging from the Early Ordovician to Cole’s (2017) phylogenetic analysis of Ordovician camerates present-day marine communities. The primary apomorphies revealed Moore and Laudon’s (1943) Diplobathrida required differentiating pentacrinoids from camerates relate to their revision. To achieve monophyly of diplobathrids while retaining distinctive posterior plating patterns, the degree of calyx plate much of Moore and Laudon’s (1943) taxonomic content, all suturing, and oral region rigidity (‘tegmen’ terminology here is dicyclic taxa equally related to both monobathrid and diplobathrid from Ausich and Kammer, 2016). Posterior plates among camerates sensu Cole (2017) are removed from the Diplobathrida pentacrinoids display a proximal relationship with the C-ray (e.g., Eknomocrinus, Reteocrinids, etc.). Following Cole’s (2017) radial plate (Guensburg, 2010; Wright, 2015a). Subclades suggested revision, our stem-based definition stabilizes the long- within the Pentacrinoidea express this affinity differently (cf. held hypothesis that monobathrids and diplobathrids represent Cladida and Disparida), and extant crinoids do not retain sister clades (Moore and Laudon, 1943; Cole, 2017). posterior plates as adults. However, the ontogenetic trajectory of Diplobathrids range from the Ordovician through lower posterior plate development in extant crinoids is tightly linked Carboniferous (Serpukhovian). Cole’s (2017) discussion on the with morphologic patterns among their Paleozoic precursors taxonomic distribution of diplobathrid morphologies suggests (Wright, 2015a). Pentacrinoid calyx plates are less closely they are generally characterized by a combination of character sutured (i.e., ankylosed) than camerates and typically have a states, including a dicyclic calyx, a concave calyx base either non-rigid to flexible oral region. In many pentacrinoids, the concealing or partially concealing the infrabasal plates, and the mouth is directly exposed on the oral surface rather than beneath presence of additional plates interrupting the radial circlet in all a tegmen (Ausich and Kammer, 2016). interrays (e.g., Rhodocrinites). Some diplobathrids sensu Cole There are several other morphologic features less diag- (2017), such as the Dimerocrinitidae, are similar to monobathrids nostic than those described above but still useful for distinguish- in having their radial circlet interrupted only in the posterior ing most pentacrinoid species from camerates. For example, interray but can easily be distinguished by their dicyclic calyx. some basal pentacrinoids such as Apektocrinus, Aethocrinus, A closer examination of post-Ordovician species indicates a and Alphacrinus incorporate additional plates within the calyx substantial revision of subclades within the Diplobathrida is (similar to camerates). However, the overwhelming majority of needed and additional research is currently underway (Cole, 2015) pentacrinoid clades do not. A major exception occurs among flexible crinoids, but flexibles are a derived group of Pentacrinoidea Jaekel, 1918 pentacrinoids and can be differentiated from camerates by other apomorphies (see Flexibilia below). Similarly, eucamerate Definition.—The Pentacrinoidea is stem-defined and as the most crinoids have pinnules, but most early to middle Paleozoic inclusive clade containing Apektocrinus ubaghsi Guensburg pentacrinoids do not. Pinnulation evolved at least once (and and Sprinkle, 2009 and Pentacrinites fossilis Blumenbach, 1804 probably several times) during the middle to late Paleozoic

among the subclade Cladida (Wright, 2015b), but these taxa Ausich et al., 2015; Wright, 2017) and contains all species can readily be distinguished from eucamerates in having a closer to Synbathocrinus than the cladid Dendrocrinus. Given pentacrinoid-like posterior plating pattern and free arms above the similar topologies across these studies, the Clade Disparida the radials. retains taxa traditionally placed within disparids (sensu Moore and Laudon, 1943) except for the hybocrinids. Inadunata Wachsmuth and Springer, 1885 A major synapomorphy and useful diagnostic trait of disparid crinoids is the presence of a single circlet of plates Definition.—The Inadunata is node-defined as the least below the radials. All other pentacrinoids are either dicyclic inclusive clade containing Synbathocrinus conicus Phillips, (cladids), pseudomonocyclic (hybocrinids) (see Sprinkle, 1836 and Dendrocrinus longidactylus Hall, 1852. 1982b), or otherwise phylogenetically distant from disparids (some derived articulates may not develop infrabasals, see Remarks.—Wachsmuth and Springer (1885) placed Lahaye and Jangoux, 1987). Disparids also have simple or non-articulate fossil crinoids with free arms above the radial compound radial plates, typically lack pinnules, and have plates within the Inadunata. Subsequent classifications divided approximate bilateral symmetry between rays oriented in one the Inadunata into the Cladida and Disparida according to the of several possible planes (see Moore et al., 1978b). As number of circlets in the calyx (Moore and Laudon, 1943; pentacrinoids, disparids have posterior plates in a proximal Moore and Teichert, 1978). In a pioneering study on phylo- position to the C ray but differ from cladids in having plates genetic approaches to crinoid classification, Simms and positioned above rather than below or in line with the C-ray Sevastopulo (1993) pointed out the Inadunata of Moore and radial plate. However, posterior plate homologies among Teichert (1978) was paraphyletic and recommended the name disparids and between inadunate clades are presently obscured be abandoned. In addition, Simms and Sevastopulo’s (1993) by a set of descriptive terms opaque to homology. Whether the revision resolved the paraphyly of cladid inadunates by proximal C-ray posterior plate is an anibrachial,’ a ‘radianal,’ an including the Flexibilia and Articulata within the Cladida. ‘anal X,’ a ‘superradial,’ or a ‘radial’ is uncertain (Moore, 1962; The division between the Camerata and Pentacrinoidea Moore and Teichert, 1978; Ausich, 1996). Future work is (discussed above) indicates disparids and cladids are more needed to help clarify primary posterior plate homologies closely related to one another than to camerates (Fig. 2). Indeed, among disparids and between cladids and disparids. The results recent phylogenetic analyses of Ordovician crinoids recover a of Wright’s (2017) analysis of Ordovician through Devonian sister group relationship between disparids and cladids (sensu pentacrinoid taxa support Guensburg’s (2010) assessment of Moore and Laudon, 1943), with hybocrinids nested within the Alphacrinus as a lower Tremadocian crinoid phylogenetically Cladida (Fig. 2) (Guensburg, 2012; Ausich et al., 2015; Wright, close to the base of the disparid clade. Guensburg (2010) 2017). Our definition of the Inadunata combines Wachsmuth considered the posterior of Alphacrinus to express a transitional and Springer’s (1885) original concept with Simms and form between ‘typical’ pentacrinoid posterior plates and Sevastopulo’s (1993) revision of the Cladida to include flexibles the ray-like extensions common among disparid taxa. A and articulates. Note that this definition places stemward re-examination of the posterior interray of basal taxa combined pentacrinoids, such as Apektocrinus, outside the Inadunata. with studies on disparid ontogeny may help resolve this issue. We combine a node-based definition of the Inadunata with stem-based definitions for the subclades Disparida and Cladida Cladida Moore and Laudon, 1943 to form a node-stem triplet to increase the stability of sister relationships between these taxa (Sereno, 1999). Definition.—The Cladida is stem-defined as the most inclusive The Clade Inadunata ranges from the Early Ordovician to clade containing Dendrocrinus longidactylus Hall, 1852 but not the present and are as a whole well characterized by Wachsmuth Synbathocrinus conicus Phillips, 1836. and Springer’s (1885) general concept of crinoids with free arms above the radial plates. Exceptions to this diagnosis occur Remarks.—The Cladida were originally defined by Moore and but are mostly restricted to a few stemward taxa and the Laudon (1943) to comprise a tremendously diverse and long- Flexibilia, which represent a derived group of inadunates ranging (Ordovician–Triassic) assemblage of dicyclic inad- (Springer, 1920). unates with their mouths covered with primary peristomial cover plates (Ausich and Kammer, 2016). Moore and Laudon’s Disparida Moore and Laudon, 1943 (1943) original concept and taxonomic content of the Cladida is paraphyletic, as they agreed with Springer’s (1920) earlier Definition.—The Disparida is stem-defined as the most inclu- assessment that flexible crinoids were more closely related to sive clade containing Synbathocrinus conicus Phillips, 1836 but some cladids than others but did not place the Flexibilia within not Dendrocrinus longidactylus Hall, 1852. the Cladida. Moreover, post-Paleozoic crinoids within Miller’s (1821) Articulata have long been considered descendants of Remarks.—Disparids comprise a diminutive but morphologi- Paleozoic cladids (Jaekel, 1918; Moore et al., 1952; Rassmussen, cally and taxonomically diverse clade of fossil crinoids ranging 1978; Simms, 1988). Simms and Sevastopulo (1993) conducted from the Ordovician through Permian. Moore and Laudon a cladistic analysis of Paleozoic cladids, flexibles, and articulate (1943) erected the Disparida to include all monocyclic crinoids and subsequently remedied cladid paraphyly by placing inadunates. Disparid monophyly is well supported by phyloge- the Flexibilia and the Articulata within the Cladida (sensu netic analyses of Ordovician crinoids (Guensburg, 2012; Moore and Laudon, 1943). Although many authors have

followed Simms and Sevastopulo’s (1993) interpretation of and Carabocrinus radiatus Billings, 1857 but not Hybocrinus relationships among these taxa, only a few authors have since conicus Billings, 1857. followed their revised rank-based classification (e.g., Brower, 2001, 2002; Donovan and Harper, 2003). Remarks.—The Porocrinida comprise a small clade of Ordovi- Our stem-based definition of the Cladida is similar in cian porocrinoids with apomorphic endothecal and/or exothecal taxonomic content to Simms and Sevastopulo’s(1993)becauseit respiratory structures. Sprinkle (1982a) pointed to many includes all species closer to Dendrocrinus than to the disparid similarities among Carabocrinus, Palaeocrinus, and the Synbathocrinus. Thus, the Cladida spans the Ordovician to the Porocrinidae and hypothesized they may be closely related. Recent and contains the major subclades Porocrinoidea, Flexibilia, Ausich et al. (2015) recovered a topology supporting this and Articulata. Cladids are most easily distinguished from their hypothesis with the euspirocrinid Illemocrinus as their sister sister group, the Disparida, in typically having a dicyclic calyx and taxon. However, Wright (2017) recovered Euspirocrinus out- posterior plates (as adults or during development) located below side the porocrinid clade within a different clade of ‘cyatho- and/or in line with the radial plate circlet (Wright, 2015a). Lastly, crine’ grade cladids. Thus, Illemocrinus is tentatively placed many middle Paleozoic to Recent cladids have pinnules, whereas within the Porocrinida, but other taxa within the Euspriocrinidae most disparids do not (Frest et al., 1979). should not be placed within the Porocrinida at this time as additional revisions are necessary. Guensburg (2012) recovered Porocrinoidea Wright, 2017 a similar tree to Ausich et al. (2015) that suggested Perittocrinus may be also be a porocrinid. fi — fi De nition. The Porocrinoidea is node-de ned as the least The stem-based definition of the Porocrinida makes them inclusive clade containing Carabocrinus radiatus Billings, sister to the Hybocrinida and retains the taxonomic membership of 1857 and Hybocrinus conicus Billings, 1857. this clade recovered in Ausich et al. (2015) and Guensburg (2012). Porocrinids can easily be distinguished from hybocrinids in Remarks.—In their description of crinoids belonging to having a dicyclic calyx and the presence of thecal respiratory Bather’s (1899) ‘Cyathocrinina’, Moore and Laudon (1943) structures (Kesling and Paul, 1968; Sprinkle, 1982a). speculated that ‘primitive’ cyathocrinids such as Carabocrinus might be closely related to the enigmatic taxon Hybocrinus. Hybocrinida Jaekel, 1918 Sprinkle (1982b) argued the stem and calyx morphology of Hybocrinus suggested hybocrinids were ‘pseudomonocyclic’ Definition.—The Hybocrinida is stem-defined as the most and listed a number of characters linking hybocrinids with cla- inclusive clade containing Hybocrinus conicus Billings, 1857 dids. Although hybocrinids have not traditionally been classi- and Hybocystites problematicus Wetherby, 1880 but not fied within the Cladida, many phylogenetic analyses of Porocrinus conicus Billings, 1857 and Carabocrinus radiatus Ordovician crinoids have recovered a clade of ‘cyathocrine’ Billings, 1857. grade cladids and hybocrinids (Guensburg, 2012; Ausich et al., 2015; Wright, 2017). Wright’s (2017) phylogenetic analysis of Remarks.—Hybocrinids comprise a small yet morphologically Ordovician through Devonian pentacrinoids recovered a clade disparate clade of Ordovician crinoids. Although the mono- comprised of Porocrinus, Carabocrinus, and the hybocrinids cyclic hybocrinids have been either considered disparids or Hybocrinus and Hybocystites. Notably, this clade is stemward classified outside the Inadunata (Moore and Laudon, 1943; of the split between flexible and other cladid crinoids. Thus, Moore and Teichert, 1978; Ausich, 1998b), Sprinkle (1982b) Wright (2017) proposed the name ‘Porocrinoidea’ to encompass suspected hybocrinids might be ‘pseudomonocyclic’ and this early diverging and morphologically unique clade of potentially related to ‘cyathocrine’ cladids (see Sprinkle, 1982a, Ordovician crinoids. 1982b). Phylogenetic analyses by Guensburg (2012), Ausich Our node-based definition of the Porocrinoidea sets up a et al. (2015), and Wright (2017) all support the monophyly of node-stem triplet that stabilizes the sister clade relationship the Hybocrinida and their sister group relationship with taxa among the Porocrinida and Hybocrinida recovered by Ausich placed in the Porocrinida (see Sprinkle, 1982b). et al. (2015), which had denser taxon sampling of Ordovician In addition to having a pseudomonocyclic calyx (infra- crinoids than Wright (2017). The Clade Porocrinoidea is likely basals absent), hybocrinids are characterized by a number of limited to the Ordovician Period, but additional analyses unusual apomorphies that distinguish them from Porocrinids sampling younger species are needed to test the extent of their (and all other crinoids). Many of these traits are similar to those geologic duration. Porocrinoids are a subclade of cladids typically present in blastozoan echinoderms, including reduc- characterized by globose, conical, or ovate calyces that possess tion in the number of arms, modification of food-gathering a number of apomorphies convergent with blastozoan echino- appendages to be recumbent (sometimes extending downward derms, such as having thecal respiratory structures, reduction in over calyx plates), and reduction in the number of calyx plates arm number and calyx plates, and/or recumbent ambulacra (see (Sprinkle and Moore, 1978). Moore and Teichert, 1978; Sprinkle, 1982a, 1982b). Flexibilia Zittel, 1895 Porocrinida Miller and Gurley, 1894 Definition.—The Flexibilia is stem-defined as the most inclu- Definition.—The Porocrinida is stem-defined as the most sive clade containing Taxocrinus macrodactylus (Phillips, inclusive clade containing Porocrinus conicus Billings, 1857 1841) but not Dendrocrinus longidactylus Hall, 1852.

Remarks.—Flexible crinoids are a morphologically homo- recent common ancestor with Dendrocrinus and Pentacrinites geneous clade that originated sometime during the Middle to than with Taxocrinus. Thus, the stem-defined clades Flexibilia Late Ordovician and range through the Permian. Springer and Eucladida are sister to one another and articulates are nested (1911, 1920) was the first to recognize that flexible crinoids within the Eucladida. This Eucladida retains much of the were closely related to inadunates. In his comprehensive 1920 meaning and taxonomic content of Moore and Laudon’s (1943) monograph, The Crinoidea Flexibilia, Springer compared concept for Paleozoic cladids while eschewing paraphyly. morphologic characteristics of the inadunate Cupulocrinus with In the Treatise on Invertebrate Paleontology, Moore et al. the earliest known flexible Protaxocrinus, citing numerous (1978a) recognized three rank-based taxa within the Cladida: similarities in calyx plating, interradial areas, and the the Dendrocrinida, the Cyathocrinida, and the Poteriocrinida. arrangement of posterior plates. Springer (1920) concluded However, it has long been questioned whether these taxa Cupulocrinus was potentially a transitional fossil that linked represent monophyletic groups (McIntosh, 1986, 2001; inadunates with flexibles, stating, “there is clearly an inter- Sevastopulo and Lane, 1988; Kammer and Ausich, 1992, mingling of the characters … and it is evident that in Cupulo- 1996; Simms and Sevastopulo, 1993; Wright, 2015a, 2015b; crinus we have to deal with a transition [sic] form whose exact Wright, and Ausich, 2015). Indeed, the Poteriocrinida is status is difficult to decide” (Springer, 1920, p. 89). Subsequent depicted in the Treatise as a polyphyletic group (Moore et al., taxonomic treatments have also recognized Cupulocrinus as 1978a, fig. 412). A phylogenetic analysis of Ordovician through occupying a proximal position to the base of the flexible tree Devonian pentacrinoids by Wright (2017) has confirmed the (Moore and Laudon, 1943; Moore and Teichert, 1978). doubts over the monophyly of these taxa. Much of the problem Phylogenetic analyses sampling flexible and other crinoid arises from ambiguous and/or uninformative apomorphies taxa have invariably recovered tree topologies supporting chosen for these taxa that perpetuate taxonomic anarchy via Springer’s (1911, 1920) hypothesis, with Cupulocrinus recov- ‘undiagnostic diagnoses’ (Wright, 2015b; see Lane, 1978, ered as the sister taxon to the Flexibilia (Brower, 1995, 2001; p. T295). Although much revision is needed, recent analyses Ausich, 1998b; Ausich et al., 2015; Wright, 2017). Wright’s indicate there is nevertheless considerable phylogenetic struc- (2017) analysis used Bayesian methods to estimate the ture among subclades of Paleozoic cladids, and additional work probability of Cupulocrinus being ancestral (sensu Foote, is under way to revise this diverse group (Wright, 2015b). 1996) to the flexible clade. Results strongly support Cupulocri- nus as occupying an ancestral position (posterior probability = Articulata Miller, 1821 0.99) (Wright, 2017). Given these results and our stem-based definition of the Flexibilia, species of Cupulocrinus are now Definition.—The Articulata is node-defined as the least inclu- placed within the flexibles. sive clade containing Endoxocrinus parrae (Gervais, 1835) and Flexible crinoids have loosely sutured calyx plating and a Antedon bifida (Pennant, 1777). remarkably uniform set of apomorphies relative to other crinoid clades. For example, flexibles differ from cladids in having Remarks.—The Articulata was proposed by Miller (1821) and interradial and intrabrachial plates and differ from other dicyclic has since developed a longstanding reputation as a problematic crinoids in typically having their lowermost circlet comprised of group that lacks a concise and unambiguous definition three (rather than five) infrabasal plates. One infrabasal plate, (Rasmussen, 1978; Simms, 1988; Simms and Sevastopulo, the ‘azygous’, is smaller than the other two, and is located in the 1993; Webster and Jell, 1999; Rouse et al., 2013). Although all C ray (except for the derived Forbesiocrinus). Many flexibles extant crinoids are invariably recognized as articulates, much retain posterior plate arrangements similar to other cladids, but confusion surrounds the recognition of fossil articulates and the posterior plates are sometimes absent in more derived flexibles. timing of their origin. The primary difficulties surround which In contrast with cladids, arms of flexible crinoids are universally apomorphy (or combination of apomorphies) is useful for uniserial and lack pinnules, and the stem is nearly always diagnosing the Articulata. For example, it is widely appreciated transversely circular (Springer, 1920). that no apomorphy or unique set of apomorphies can presently diagnose fossil articulates without ambiguity (Simms, 1988; Eucladida Wright, 2017 Simms and Sevastopulo, 1993; Webster and Jell, 1999; Rouse et al., 2013). Most crinoid workers have obviated this problem Definition.—The Eucladida is stem-defined as the most inclu- by simply treating the Articulata as synonymous with post- sive clade containing Dendrocrinus longidactylus Hall, 1952 Paleozoic crinoids (see Simms and Sevastopulo, 1993). and Pentacrinites fossilis Blumenbach, 1804 but not Taxocrinus However, this usage is problematic because this definition is not macrodactylus (Phillips, 1841). based on any explicit phylogenetic hypothesis. Moreover, many Paleozoic groups of fossil cladids share different combinations Remarks.—The revision of the Cladida to be monophyletic of traits typically listed as ‘diagnostic’ for the Articulata requires placing the subclades Porocrinoidea, Flexibilia, and (Webster and Jell, 1999; Webster and Lane, 2007). If the con- Articulata within a more inclusively defined Clade Cladida cept of what defines the Articulata depends on the choice of a (Simms and Sevastopulo, 1993; Wright, 2017). However, the particular combination of apomorphies alone, then questions Cladida (sensu Moore and Laudon, 1943) is traditionally con- regarding the ‘origin of the Articulata’ will always depend on ceived as a Paleozoic-age paraphyletic group that excludes the which specific combination was chosen a priori to be diagnostic. Flexibilia. The Eucladida was proposed by Wright (2017) to Without a phylogenetic definition, it is impossible to objectively comprise all species within the Clade Cladida sharing a more specify a precise set of synapomorphies for the Articulata. Thus,

we propose herein to define the Articulata as the crinoid crown classification. The use of intermediate ranks (e.g., Parvclass) group containing the last common ancestor of the extant follows traditional use in pre-existing taxonomic literature isocrinid Endoxocrinus parrae and the comatulid Antedon (see Carroll, 1988; Sibley, 1994; Benton, 2005). Two older bifida, and all of its descendants. taxonomic names, the Pentacrinoidea Jaekel, 1918 and Inad- As discussed by Ruta et al. (2003), the concepts of stem unata Wachsmuth and Springer, 1885, are formally reinstated groups and crown groups are sometimes misinterpreted or herein because they represent meaningful clades as described misused in the paleontological literature. Used properly, crown above. groups are defined by extant taxon specifiers. Notably, crown Post-Ordovician cladids (sensu Moore and Laudon, 1943) groups may be comprised of many (or mostly) extinct fossil and the Protocrinoida (Guensburg and Sprinkle, 2003) remain species. For example, if a fossil crinoid is more closely related to problematic groups. Because the rank for a monophyletic some extant species than others, it is a member of the crown Cladida must be above flexibles and articulates (Simms and group. According to Rouse et al. (2013), the most recent Sevastopulo, 1993), we propose the name Cyathoformes to common ancestor of all extant crinoids lived sometime during contain taxa traditionally placed within the Cladida that are the Middle to Upper Triassic. Thus, our node-based definition sister to the Articulata. Relationships among these taxa are the eliminates the non-phylogenetic concept of ‘post-Paleozoic subject of future phylogenetic research (Wright, 2015b) and Crinoidea’ while retaining the majority of post-Paleozoic are not treated further here. From their initial description crinoids traditionally included within the Articulata. The Clade (Guensburg and Sprinkle, 2003), the protocrinoids have been an Articulata is synonymous with the Crown Crinoidea (Sumrall, important but confounding group of crinoids that display char- 2014), and we advocate workers use these terms interchange- acteristics of both crinoids and other stalked echinoderms. ably depending on context (e.g., discussing relationships among Guensburg and Sprinkle (2003) regarded the protocrinoid as an crinoids or between crinoids and non-crinoids). Traits that may “order (plesion)”. The validity of the protocrinoids was later be present in the Articulate ancestor are listed in Simms (1988), questioned by Guensburg and Sprinkle (2009) and led Simms and Sevastopulo (1993), Webster and Jell (1999), and Guensburg (2012) to formally place them within the Camerata. Rouse et al. (2013). However, Ausich et al. (2015) recovered a sister group The Articulata likely contains most post-Paleozoic taxa relationship between Titanocrinus and Glenocrinus, but with traditionally considered articulates, including the ~600 or so protocrinoids more closely related to non-camerates than cam- extant species. Although we define Articulata with precision and erates. In contrast, Cole’s (2017) analysis of Ordovician crinoids phylogenetic stability (Rouse et al., 2013, 2015), it remains recovered the protocrinoids as more closely related to camerates difficult in practice to unambiguously identify fossil articulates, than non-camerates. Thus, we have carefully chosen our clade particularly among specimens near the base of the articulate definitions to not depend on a particular phylogenetic hypoth- tree. However, such difficulties are already present and have esis or morphologic interpretation of these significant but long obfuscated the origin of the crinoid crown group. The more problematic taxa. For the moment, we tentatively place both important problem is resolving the phylogenetic position of the protocrinoid taxa as Crinoidea incertae sedis subclass common ancestor of extant crinoids within the myriad of fossil Protocrinoida. lineages. Our definition provides a useful framework for future In our present understanding of crinoid evolution, the first phylogenetic research to uncover relationships between poten- major divergence occurs between camerates and all other cri- tial stem articulates, extinct crown group lineages, and extant noids (Fig. 2). The subclass rank is retained for the Camerata; species. and the subclass Pentacrinoidea Jaekel, 1918 is proposed for its sister group (Table 2). Within the Camerata, the orders A revised rank-based classification of the Crinoidea Diplobathrida and Monobathrida are retained as sister groups, and the infraclass Eucamerata Cole (2017) unites these two Crinoid clades identified herein confirm many long-held views orders. In phylogenetic analyses of camerates, several taxa are on the major divisions among crinoids from both the founda- not placed within the Monobathrida and Diplobathrida (sensu tional work of Moore and Laudon (1943) and Moore and Cole, 2017). Thus, they are considered here to be stem euca- Teichert (1978) to more recent analyses (i.e., Ausich, 1998a, merates (see remarks for Eucamerata above). The subclass 1998b; Guensburg and Sprinkle, 2003; Guensburg, 2012; Camerata unites these stem taxa with eucamerates. Ausich et al., 2015). Results from all of these studies recognized In terms of species richness, the subclass Pentacrinoidea is the Camerata, Diplobathrida, Monobathrida, Hybocrinida, the largest crinoid clade. This includes the Disparida, Cladida, Disparida, Cladida, and Flexibilia. The challenge is to represent Hybocrinida, and Articulata of Moore and Teichert (1978), these widely recognized clades in a rank-based Linnaean clas- which coincides exactly with Jaekel’s (1918) concept of the sification scheme that maximizes common usages of names for Pentacrinoidea (see Lane, 1978). Hence, we have proposed the crinoid lineages (Moore and Teichert, 1978) and is consistent reinstatement of this name. The Pentacrinoidea is comprised of with a phylogenetic understanding of relationships (Wiley and the infraclass Inadunata and their stem taxa (e.g., Apektocrinus). Lieberman, 2011). In our revision, the Crinoidea remain a class The concept for the Inadunata in Moore and Teichert (1978) and every attempt is made to retain orders as recognized in united the Disparida and the Cladida. Here, the infraclass Moore and Teichert (1978). Unfortunately, the tree topology of Inadunata unites the Disparida, Cladida, and all of their Figure 2 prevented the attainment of the latter in all instances, descendants. This usage circumvents the non-phylogenetic but the addition of intermediate Linnaean ranks makes it easier usage of the Inadunata (sensu Moore and Teichert, 1978) and is to apply a phylogenetic perspective to rank-based crinoid consistent with the phylogenetic conclusions of Simms and

Table 2. Revised rank-based classification of the Crinoidea. on the historic subdivision of crinoids into major lineages. It is Class Crinoidea Miller, 1821 hoped that the phylogenetic classification schemes presented †Subclass Camerata Wachsmuth and Springer, 1885 herein will help provide a framework for future research on ‘Stem eucamerates’ (e.g., Eknomocrinus) Infraclass Eucamerata Cole, 2017 crinoid phylogeny and offer guidance to crinoid workers and Order Diplobathrida Moore and Laudon, 1943 non-specialists alike interested in using this fascinating group of Order Monobathrida Moore and Laudon, 1943 Crinoidea incertae sedis: †Protocrinoidea Guensburg and Sprinkle, 2003 echinoderms to study evolutionary patterns and processes. Subclass Pentacrinoidea Jaekel, 1894 †‘Stem inadunates’ (e.g., Apektocrinus) Infraclass Inadunata Wachsmuth and Springer, 1885 †Parvclass Disparida Moore and Laudon, 1943 Acknowledgments Order Eustenocrinida Ulrich, 1925 Order Maennilicrinida Ausich, 1998b Order Tetragonocrinida Stukalina, 1980 We thank the organizing committee of the 2015 Progress Order Calceocrinida Meek and Worthen, 1869 in Echinoderm Paleobiology conference and guest editors Disparida incertae sedis: ‘Homocrinida’ Kirk, 1914 Disparida incertae sedis: ‘Myelodactyla’ Miller, 1883 S. Zamora and I.A. Rahman for inviting us to submit this con- Disparida incertae sedis: ‘Pisocrinoidea’ Ausich and Copper, 2010 tribution. G.D. Sevastopulo and an anonymous reviewer are Parvclass Cladida Moore and Laudon, 1943 thanked for their helpful reviews and useful comments. †Superorder Porocrinoidea Wright, 2017 Order Porocrinida Miller, and Gurley, 1894 I.A. Rahman provided careful editorial assistance and made Order Hybocrinida Jaekel, 1918 several suggestions that improved the manuscript. This research †Superorder Flexibilia Zittel, 1895 (Cupulocrinus d’Orbigny, 1849) Order Taxocrinida Springer, 1913 was supported by grants from numerous organizations, includ- Order Sagenocrinida Springer, 1913 ing the National Science Foundation (DEB 1036416, WIA), the Magnorder Eucladida Wright, 2017 ’ †Superorder Cyathoformes new superorder Paleontological Society s N.G. Lane award (DFW, SRC), Cyathoformes incertae sedis: ‘Cyathocrinida’ Bather, 1899 Sigma Xi (DFW, SRC), Palaeontological Association (DFW, Cyathoformes incertae sedis: ‘Dendrocrinida’ Bather, 1899 Cyathoformes incertae sedis: ‘Poteriocrinida’ Jaekel, 1918 SRC), the American Federation of Mineralogical and Eucladida incertae sedis: †‘Ampelocrinida’ Webster and Jell, 1999 Geological Societies Student Scholarship (DFW, SRC), and Superorder Articulata Miller, 1821 The Ohio State University School of Earth Sciences’ Friends of †Order Holocrinida Jaekel, 1918 Rasmussen, 1978 †Order Encrinida Matsumoto, 1929 Orton Hall Scholarship (DFW, SRC). DFW was also supported †Order Millericrinida Sieverts-Doreck, 1953 by an Ohio State University Presidential Fellowship. †Order Uintacrinida Zittel, 1879 †Order Roveacrinida Sieverts-Doreck, 1953 Order Cyrtocrinida Sieverts-Doreck, 1953 Order Hyocrinida Rasmussen, 1978 Order Isocrinida Sieverts-Doreck, 1953 References Order Comatulida Clark, 1908 Alaniz, R.J., 2014, Dredging evolutionary theory: The emergence of the deep sea as a transatlantic site for evolution 1853–1876 [Ph.D. dissertation]: San Diego, University of California, 360 p. Sevastopulo (1993). With the Inadunata an infraclass, the Angelin, N.P., 1878, Iconographia Crinoideorum in stratis Sueciae Siluricis Disparida and Cladida are both parvclasses. Taxa placed within fossilium: Holmiae, Samson and Wallin, 62 p. 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